This application is based on Japanese Patent Application Nos. 2000-46473 filed Feb. 23, 2000 and 2000-124853 filed Apr. 25, 2000, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
This invention relates to a magnetic recording medium, which is used in various magnetic recording medium devices including external storage devices of computers and AV-HDD, and a method for producing the magnetic recording medium.
2. Description of the Related Art
Various compositions and structures of a magnetic layer and various materials for a nonmagnetic undercoat layer and a seed layer have been proposed for magnetic recording media for which a higher recording density and a lower noise have rapidly been demanded in recent years. Particularly in recent years, a proposal has been made for a magnetic layer, generally called a granular magnetic layer, which has a structure comprising magnetic crystal grains surrounded with a nonmagnetic nonmetallic substance such as an oxide or a nitride.
For example, Japanese Patent application Laid-open No. 8-255342(1996) describes that a low noise is achieved by laminating a nonmagnetic film, a ferromagnetic film, and a nonmagnetic film sequentially on a nonmagnetic substrate, and then heat-treating the laminate to form a granular recording layer having ferromagnetic crystal grains dispersed in the nonmagnetic film. In this case, cobalt or an alloy composed mainly of cobalt is used as the magnetic layer, and a metal, an oxide, a nitride, carbon or a carbide is used as the nonmagnetic film. U.S. Pat. No. 5,679,473 describes that a granular recording layer having a structure comprising magnetic crystal grains surrounded with a nonmagnetic oxide and thereby individually separated can be formed by performing RF (radio frequency) sputtering with the use of a CoNiPt target having an oxide, such as SiO2, added thereto, and that high Hc (coercive force) and low noise are realized by this recording layer.
Such a granular magnetic layer has been considered to obtain low noise characteristics for the following reason: A nonmagnetic nonmetallic grain boundary phase physically separates the magnetic grains. Thus, the magnetic interaction between the magnetic grains lowers to suppress the formation of zigzag domain walls occurring in a transition region of recording bits.
The causes of the noise in a recording medium are the size of magnetic grains constituting the medium, and fluctuations in magnetization due to magnetic interaction between the grains. To maintain high SNR consistent with an increased recording density, it is necessary to keep the number of magnetic grains per bit cell at a certain value or higher, namely, to make the magnetic grains finer. However, in a state in which a great exchange interaction works between the magnetic grains, finer crystal grains do not necessarily mean finer magnetization reversal units. Thus, in order to make the magnetization reversal unit (expressed as an activation magnetic moment) itself small, it is also necessary to suppress the exchange interaction between the grains. In making the grains finer, moreover, it is necessary to impart a certain magnitude of magnetic anisotropy energy to the magnetic grains themselves so that magnetic characteristics essential for high resolution recording (great Hc/Mrt) can be obtained without super-paramagnetism. A granular structure comprising magnetic grains with high magnetic anisotropy energy dispersed in a nonmagnetic matrix is aimed at fulfilling all of the above-described strict requirements for high SNR.
A conventionally used CoCr-based metallic magnetic layer is formed at a high temperature. Thus, Cr is segregated from Co-based magnetic grains and precipitated into the grain boundary to decrease the magnetic interaction between the magnetic grains. In the case of the granular magnetic layer, on the other hand, a nonmagnetic nonmetallic substance is used as the grain boundary phase. Thus, the advantage is obtained that Cr is segregated more easily than Cr in the conventional magnetic layer, whereby isolation of the magnetic grains can be promoted relatively easily. In particular, with the conventional CoCr-based metallic magnetic layer, raising the substrate temperature during film formation to 200xc2x0 C. or higher is absolutely necessary for sufficient segregation of Cr. The granular magnetic layer, by contrast, is advantageous in that even during film formation without heating, the nonmagnetic nonmetallic substance undergoes segregation.
However, a magnetic recording medium having a granular magnetic layer requires that a relatively large amount of Pt be added to a Co alloy in order to realize the desired magnetic characteristics, especially, high coercive force Hc. The aforementioned U.S. Pat. No. 5,679,473 also needs expensive Pt as much as 11 at % in order to achieve Hc of about 2400 Oe. To realize comparable Hc by use of the conventional CoCr-based metallic magnetic layer, on the other hand, the amount of Pt required is as small as 5 at %. Generally, with granular magnetic layer Pt in an amount as large as 16 at % is needed to realize Hc of 2800 Oe. With the conventional CoCr-based metallic magnetic layer, on the other hand, the amount of Pt required is only 8 at %. In recent years, with the increase in magnetic recording density, there has been an increasing demand for Hc as high as 3,000 Oe or more. The granular magnetic layer, which requires a large amount of expensive Pt, is posing the problem of increasing the manufacturing cost. A further decrease in the medium noise is also demanded in accordance with the increase in the density. The need for meticulous control of the magnetic crystal grain size of the granular magnetic layer and a fine structure such as a segregation structure is increasing.
Extensive studies have been conducted to achieve high Hc, a low cost, and a further decrease in noise for a granular magnetic layer. These studies have clarified that high Hc and a low medium noise can be achieved without an increase in the consumption of expensive Pt, by forming a nonmagnetic metallic intermediate layer between a granular magnetic layer and a nonmagnetic undercoat layer, the nonmagnetic metallic intermediate layer comprising a nonmagnetic metal or an alloy thereof and having a crystal structure which is a hexagonal close-packed (hcp) structure.
More preferably, the nonmagnetic metallic intermediate layer is two-layered. As a result, it has been found that the mean crystal grain diameters in the magnetic layer and their variations can be made small, and more favorable results can be obtained.
Also, the use of the nonmagnetic metallic intermediate layer gives a high Hc easily. Thus, a substrate need not be heated during film formation of a medium according to the present invention. Furthermore, the manufacturing process can be simplified and performed for a low cost, and an inexpensive plastic can be used as a substrate, in addition to a conventional Al or glass substrate.
In the first aspect of the present invention, a magnetic recording medium comprising at least a nonmagnetic undercoat layer, a nonmagnetic metallic intermediate layer, a magnetic layer, a protective film, and a liquid lubricant layer sequentially laminated on a nonmagnetic substrate comprises:
the magnetic layer comprising crystal grains having ferromagnetism and nonmagnetic grain boundaries surrounding the crystal grains, and
the nonmagnetic metallic intermediate layer comprising at least one layer, and a crystal structure of each layer being a hexagonal close-packed structure.
Here, the nonmagnetic metallic intermediate layer may include a layer comprising a metal selected from the group consisting of Ti, Zr, Hf, Ti alloys, Zr alloys, and Hf alloys.
The nonmagnetic metallic intermediate layer may include a layer comprising a CoCr alloy containing 30% to 50% of Cr.
The nonmagnetic metallic intermediate layer may have a structure consisting of two different layers laminated together, and one of the layers may comprise a metal selected from the group consisting of Ti, Zr, Hf, Ti alloys, Zr alloys, and Hf alloys, and the other layer may comprise a CoCr alloy containing 30% to 50% of Cr.
The nonmagnetic metallic intermediate layer may have a structure consisting of two different layers laminated together, and a lower layer of the two layers may be a layer comprising a CoCr alloy containing 30% to 50% of Cr, and an upper layer of the two layers may be a layer comprising a metal selected from the group consisting of Ti, Zr, Hf, Ti alloys, Zr alloys, and Hf alloys.
The nonmagnetic grain boundaries in the magnetic layer may comprise at least one oxide selected from the group consisting of oxides of Cr, Co, Si, Al, Ti, Ta, Hf and Zr, and the crystal grains having ferromagnetism in the magnetic layer may comprise an alloy formed by adding to a CoPt alloy at least one substance selected from the group consisting of Cr, Ni and Ta.
The nonmagnetic undercoat layer may comprise Cr or a Cr alloy.
The nonmagnetic substrate may be selected from the group consisting of crystallized glass, chemical tempered glass, and plastics.
In the second aspect of the present invention, a method for producing a magnetic recording medium comprising at least a nonmagnetic undercoat layer, a nonmagnetic metallic intermediate layer, a magnetic layer, a protective film, and a liquid lubricant layer laminated sequentially on a nonmagnetic substrate, comprises the steps of:
laminating the nonmagnetic undercoat layer on the nonmagnetic substrate;
laminating the nonmagnetic metallic intermediate layer on the nonmagnetic undercoat layer, the nonmagnetic metallic intermediate layer having a crystal structure being a hexagonal close-packed structure;
laminating the magnetic layer on the nonmagnetic metallic intermediate layer, the magnetic layer comprising crystal grains having ferromagnetism and nonmagnetic grain boundaries surrounding the crystal grains;
laminating the protective film on the magnetic layer; and
laminating the liquid lubricant layer on the protective film, and wherein
the respective steps are performed without prior heating of the nonmagnetic substrate.
Here, the step of laminating the nonmagnetic metallic intermediate layer may include the step of providing a layer comprising a metal selected from the group consisting of Ti, Zr, Hf, Ti alloys, Zr alloys, and Hf alloys.
The step of laminating the nonmagnetic metallic intermediate layer may include the step of providing a layer comprising a CoCr alloy containing 30% to 50% of Cr.
The step of laminating the nonmagnetic metallic intermediate layer may include the step of providing a layer comprising a metal selected from the group consisting of Ti, Zr, Hf, Ti alloys, Zr alloys, and Hf alloys, and the step of providing a layer comprising a CoCr alloy containing 30% to 50% of Cr.
The step of laminating the nonmagnetic metallic intermediate layer may include the step of providing on the nonmagnetic undercoat layer a layer comprising a CoCr alloy containing 30% to 50% of Cr, and the step of providing on the layer comprising the CoCr alloy a layer comprising a metal selected from the group consisting of Ti, Zr, Hf, Ti alloys, Zr alloys, and Hf alloys.
The nonmagnetic grain boundaries in the magnetic layer may comprise at least one oxide selected from the group consisting of oxides of Cr, Co, Si, Al, Ti, Ta, Hf and Zr, and the crystal grains having ferromagnetism in the magnetic layer may comprise an alloy formed by adding to a CoPt alloy at least one substance selected from the group consisting of Cr, Ni and Ta.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.